linux-kernel - Re: [PATCH] x86/mm: Do not verify W^X at boot up

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Date:   Mon, 24 Oct 2022 09:14:45 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/mm: Do not verify W^X at boot up

On 10/24/22 08:45, Steven Rostedt wrote:
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -587,6 +587,10 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
>  {
>  	unsigned long end;
>  
> +	/* Kernel text is rw at boot up */
> +	if (system_state == SYSTEM_BOOTING)
> +		return new;

Hi Steven,

Thanks for the report and the patch.  That seems reasonable, but I'm a
bit worried that it opens up a big hole (boot time) when a W+X mapping
could be created *anywhere*.

Could we restrict this bypass to *only* kernel text addresses during
boot?  Maybe something like this:

	if ((system_state == SYSTEM_BOOTING) &&
	    __kernel_text_address(start))
		return new;

That would be safe because we know that kernel_text_address() addresses
will be made read-only by the time userspace shows up and that
is_kernel_inittext() addresses will be freed.

Long-term, I wonder if we could teach the early patching code that it
can't just use memcpy().